It is believed that capacitors capable of operating in harsh, high temperature environments including temperature ranges of between about 150 degrees Celsius (° C.) to 300° C. will be in high demand for power electronics for markets such as oil and gas well drilling. Such “down hole” applications are becoming increasingly demanding from a temperature of operation standpoint, as drilling for deeper and less accessible reserves becomes more and more the norm.
Until now, the “down hole” industry has relied on the use of multilayer ceramic capacitors (“MLCC's”) made essentially from modified barium titanate (BaTiO3). Such capacitors are manufactured to meet electronic industry standards such as X7R (wherein capacitance should not change by more than plus or minus 15% over an operating temperature range of −55° C. to 125° C.), X8R (which extends the range of the X7R to 150° C. with plus or minus 15% change), or BX (which includes an additional requirement of limiting capacitance change to −25% over the −55° C. to 125° C. temperature range with rated voltage applied). Typically, the dielectrics employed in these MLCC's are BaTiO3 based, with various donor/acceptor dopant and fluxing additions that shift and flatten the high temperature (approximately 125° C.) ferroelectric tetragonal to paraelectric cubic phase transitions (Curie point) so that the previously mentioned electronic industry's temperature coefficients can be met, as well as to improve the sintering characteristics of the individual dielectrics. Typical of such dielectric compositions are those disclosed in U.S. Pat. Nos. 6,723,673, 5,128,289 and 5,571,767. (Each of these patents includes an inventor or co-inventor that is the inventor of the present disclosure, which patents are also hereby incorporated herein by reference thereto.) The sintered dielectric of MLCC's manufactured with these types of materials exhibits what is commonly referred to as a core/shell microstructure in which nearly pure BaTiO3 grain cores are surrounded by a heavily doped shell region. It is this heterogeneous structure that allows for the shifting and flattening of the Curie point. (For purposes herein, the phrase Curie point is to mean a transition temperature marking a change in the ferroelectric properties of a substance, especially the change from a ferroelectric to a paraelectric state.)